Digital communications systems utilize communication channels over which traffic data is communicated or transported. These channels are typically bandwidth limited, having a finite channel capacity. The channel capacity together with other properties of the channel, such as various forms of noise and interference, will, with statistical certainty, cause, or otherwise result, in the injection of error conditions in the traffic data communicated over the channel. The effects of these error conditions may be particularly evident in wireless communications systems, which utilize generally unpredictable over-the-air communications channels through which remote stations communicate with a central station.
A technique for eliminating, or at least reducing, the effects of these error conditions is called Forward Error Correction (FEC). In general, the employment of an FEC technique entails transmitting error detection data and error correction data along with the bearer data. The error detection data and error correction data are typically derived from the bearer data itself by employing an error detection algorithm and error correction algorithm known to the receiver as well as the transmitter, and in the case of a digital wireless communications system, a remote station and a central station in communication with one another.
FEC techniques have been employed in Time Division Multiple Access (TDMA) wireless communications systems. TDMA systems typically allow communication between a plurality of remote stations and a central station using the same frequency band and transmitting bearer data between remote stations and the central station during discrete time periods (i.e., each remote station transmits and receives bearer data broken up into bearer data bursts during respective time slots of cyclically repeating time frames).
In a known wireless communication, prior to transmission, the central station or remote station appends or encodes the bearer data with error detection data and error correction data according to a respective error detection algorithm and error correction algorithm. The reciprocal remote station or central station receives each error correctable bearer data packet, automatically corrects any errors in each error correctable bearer data packet (within the limits of the error correction algorithm) by processing the error correctable bearer data packet according to the error correction algorithm, and detects any residual errors in each corrected error correctable bearer data packet by processing the corrected error correctable bearer data packet according to the error detection algorithm.
The use of an FEC technique to eliminate or reduce the effects of transmission errors, however, does not come without a cost to the communications system. The transmission bandwidth available to a user transmitting in a particular time slot in known systems is reduced by the overhead required to transmit the additional data, and in particular, the error correction data. The transmission of error correction data with each error correctable bearer data packet may require 100% or more overhead in some instances. This increase in overhead typically results in a reduction in the bandwidth available for the traffic data (for a fixed transmission bit rate per user).
In known wireless communications systems, the Bit Error Rate (BER) of the traffic data communicated between a central station and a remote station depends on dynamically varying conditions, such as, the relative distance between the remote station and the central station, environmental conditions, traffic data transmission rate, etc. In one system, a description of which is described in an application filed concurrently herewith, which is hereby expressly and fully incorporated herein by reference, the amount of error correction data is varied in accordance with the BER to more efficiently utilize the resources of the system. In such a case, the amount of bearer data contained in the error correctable bearer data packet transmitted and received over a particular time slot may vary. As a result, management of the bearer data can become more difficult resulting in a more complex system, and the data rate provided to the user dynamically varies with channel conditions. Many systems reserve bandwidth to accommodate peak requirements, resulting in wasted bandwidth during normal conditions.
There thus is a need for a communications system that employs an FEC scheme that results in transmission of error correctable bearer data blocks with a uniform amount of bearer data, allowing use of available unused capacity for FEC data (i.e., improving the overall throughput system usage), while still providing high quality communication.
Other preferred methods may comprise respectively transmitting bearer data and error correction data in different frequency bands (FDMA), or using different codes (CDMA), or using any other orthogonal mechanism.